Wednesday, March 11, 2009

A Universal Definition for a Planet, Moon, Sun, Etc.

To follow up on and better explain my reasoning I assert that the definition for a planet, a moon, an asteroid, comet, or the like should be one that can be applied universally, be based upon a good deal of invariable physical characteristics of matter, and make good sense to a person of a non-scientific background. Because ultimately an in any linguistic argument it is common usage that determines the winner in long run. It does not matter if some scientist asserts that "a strawberry isn't a fruit, it is pseudocarp or 'false fruit'", people will still call it a fruit and ignore the scientific definition.

To that end I think that it should be mass or mean radius that is the primary characteristic that should determine if something is a planet or not.

Of the two mass is the harder to discover about a new object unless a satellite can be observed. So for practical reasons I think it should be mean radius that determines a body's status, but with an eye towards how mass, gravity, rotation, and orbit interact to produce a more or less round body. I think the right answer, since nature does not provide a nice clear cut boundary, is a mean radius of around 1000 kilometers. Why? It's a nice round number and from what we have seen so far even a object made of fairly rigid material like stone or iron could not retain a highly irregular shape in such a circumstance. But I am open to be correct should someone with a better understanding of the math involve show that a theoretical object made entirely of iron (for example) would not round itself even with a mean radius of 1000 kilometers. Or that it should be lowered to something more like 470 km radius of Ceres.

The minor part of the definition should be that it orbits a star, brown dwarf, or is not orbiting another object that can be defined as a planet. If an object otherwise qualifies as a planet and it has only 2/3 the mean radius of larger object it is a moon. If the two objects are close in size, the larger having a radius of no more than one and a half times the the smaller, then they are binary planets. Why 2/3? Because anything less and the more massive object would be getting too close to being twice as big in radius/diameter and in pure math it would have more than three times the volume. (3.375 times the volume if you want to be precise)

This brings me to moons. I think that anything that would not otherwise qualify as a planet due to orbiting a planet is a moon and anything smaller should be a dwarf moon, minor moon, or something of that nature. That would mean in the solar system there are seven moons and about ten minor moons. Anything that would otherwise be a comet, asteroid, or some other irregular body is a natural satellite, but not a moon.

So under this definition that makes ten planets in the solar system, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Pluto, and Eris.

And the seven moons are Ganymede, Callisto, Titan, The Moon, Io, Europa, and Triton.

The minor moons would include Rhea, Titania, Oberon, Iapetus, Dione, Tethys, Umbriel, Ariel, Charon, and Enceladus.

And rather like the asteroids they so resemble the natural satellites are too numerous to pay attention to all of them.

1 comment:

Harriet said...

Well reasoned. I will have to think more on this!